The field of the present invention relates generally to improved methods of geophysical exploration. More particularly, the present invention relates to a novel method for locating hydrocarbon-containing formations surrounding a well borehole, especially in previously producing wells. The present invention also provides a method for determining changes in deposition, compaction and porosity for selected formation lithologies surrounding a well borehole.
The desirability of increasing production from previously producing wells has long been recognized. Substantial efforts have been directed in this area to increasing production from previously producing formations such as Gournay, U.S. Pat. No. 4,399,693. However, it is oftentimes desirable to reevaluate other formations surrounding the well borehole which have not previously been brought into production for the possibility of overlooked hydrocarbon-containing formations.
Present well logging techniques are somewhat restricted in their ability to reevaluate such overlooked formations surrounding the well borehole because of limitations generally associated with such well logging techniques when employed in cased well boreholes. Specifically, the steel casing can act as a barrier to or impose severe limitations on the capability of most well logging techniques to reevaluate formations surrounding the cased well borehole. Moreover, most well logging techniques used within a cased well borehole have an exploration range limited to a few inches into the surrounding formations beyond the casing, e.g., gamma-gamma ray, neutron, sonic, electric log, etc.
Accordingly, there remains a need in the exploration field for an accurate method for determining the location of hydrocarbon-containing formations overlooked during initial production. Additionally, a need exists for a method of locating overlooked hydrocarbon-containing formations surrounding a well borehole, at a reasonable cost without having to pull the casing from the well borehole having sufficient exploration range and evaluation capabilities. Moreover, a need exists to provide indicia of hydrocarbon-containing formations surrounding the cased well borehole.